The invention relates generally to a disk player used for reproducing various information from a disk in which the information has been prerecorded in the form of pits on spiral or coaxial tracks.
Recently, disks have been developed in which video and/or audio signals or the like are recorded as the intermittence of pits after converting the signal into a PCM signal, and then frequency modulating the PCM signal. In apparatus for reproducing a frequency modulated (FM) signal from such disks, the frequency modulated signal picked up by a pickup stylus is frequency demodulated to restore the original video or audio signal.
Generally speaking, in audio disks having a digital signal train, bit rate is relatively great, for instance, 6 Mbit/sec, and the frequency spectrum of the recording signal is of wide range. Therefore, when a signal having such a wide frequency spectrum is frequency modulated, the modulation index lowers especially in connection with high frequency components so that high S/N cannot be obtained.
On the other hand, with such a disk, since reproduction is effected with the rotational speed kept constant irrespective of the radial position of the pickup device, the relative linear velocity between the pickup stylus and the disk at the outer portion (first 45 minutes or so in a disk having a reproducing time length of 60 minutes per one side) is greater than that at the inner portion (last 15 minutes or so in the same disk). Therefore, the level of high frequency components decreases as reproducing the inner portion of a disk, because of the reason that the modulation index of the above-mentioned high frequency components lowers, and the reason that the linear velocity at the inner portion is small. Particularly, at the inner portion, the carrier to noise ratio of the FM signal lowers so that the level of a reproduced signal derived from the pickup lowers from the level of the original recorded signal especially at high frequencies.
For this reason, since a circuit for compensating for the level decrease of the high frequency components on reproduction at the inner portion has not been provided in conventional circuit arrangements for reproducing apparatus, distortion occurs in the output signal of a demodulator responsive to such a reproduced signal. As a result, distinguishment of reproduced data becomes difficult, resulting in errors in codes, and therefore, there is a drawback in that a signal cannot be correctly reproduced throughout the entire disk.